1. Field of the Invention
The present invention relates to a combined bearing device, for equipping the variable pitch rotors of rotorcraft and, in particular, the main or lift and rear or antitorque rotors of helicopters, of the particular type in which each blade is retained on the hub of the rotor by an elongate arm, substantially radial with respect to the axis of rotation of the rotor, rigid in the longitudinal direction so as to transmit the centrifugal forces from the blade and flexible under torsion and flexion forces so as to allow the angular flapping and drag oscillations of the blade as well as variations of its pitch, and which is surrounded without contact, as far as the vicinity of the hub, by a rigid tubular sleeve controlled externally by means of a lever controlling the pitch of the blade, and whose end the closest to the axis of the rotor is connected to the hub by the combined bearing device, which surrounds the root of the flexible arm on the hub.
In a rotor of this particular type, the combined bearing device forming the object of the invention fulfills simultaneously the functions of angle of attack bearing, of bearing for resiliently returning the corresponding blade under drag and flapping conditions, and finally of incorporated or internal bearing for damping the oscillatory movements of the blade under drag conditions.
2. Description of the Prior Art
Variable pitch and main or antitorque rotors and rotor hubs for helicopters, having the particular structure described above and equipped with a combined bearing device such as also described above are already known from the French Pats. No. 2 397 325 and 2 397 327 of the applicant.
French Pat. No. 2 397 327 describes a variable pitch rotor in which the flexible arm is an elongate connecting element formed essentially by a bundle of high mechanical strength fibers, coated individually for example with a resin and agglomerated together by a vulcanized elastomer. The outer and inner ends, with respect to the axis of rotation of the rotor, of the rigid tubular sleeve are fixed respectively to the foot of the corresponding blade and to a rigid ring on which is externally articulated the lever controlling the blade pitch. The position of the flapping axis of each blade is determined by spherical ball-joint elements, made preferaby from stratified elastomer, which are inserted respectively between rigid parts of the hub and the rigid ring fixed to the internal end of the sleeve, and frequency adapters, or dampers damping essentially the oscillating movements of the blade under drag conditions, preferably formed by elastomer plates of great stiffness and high hysteresis, are associated respectively with the spherical ball-joint elements.
In such a rotor, the action of the lever controlling the pitch of each blade on the rigid ring fixed to the tubular sleeve, for varying the angle of attack of the corresponding blade, causes a shearing force to be exerted on the stratified elastomer spherical ball-joint elements, which have a certain elastic return capacity opposing such control, as well as the angular oscillations of the blade under flapping and drag conditions whereas damping of the oscillating movements of the blade under drag conditions is provided by the plates of the frequency adapters, which are either inserted between the rigid parts of the hub and the spherical ball-joint elements, or shaped so as to be incorporated in the stratified elastomer spherical ball-joint elements.
In this French patent, this rotor structure is usable advantageously as main rotor for a light helicopter in which the bundles of fibers of the connecting elements of at least two blades form part of a single bundle of fibers, having at least one intermediate part, incorporated in the rotor hub which, perpendicularly the the plane of this rotor, has a thickness slightly greater than that of the connecting elements of the blades. In addition, the rigid parts of the hub which support the stratified spherical ball-joint elements (formed of an alternating stack of elastomers and rigid cups, made for example from metal) are formed by rigid preferably metal plates which are fixed respectively to the upper and lower faces of the hub and which extend cantilever fashion on each side of each element or connecting arm, at least as far as the level of the position chosen for the flapping axis of the corresponding blade.
In French Pat. No. 2 397 325, the flexible and torsible arm, which connects each rotor blade to the hub, and is in particular intended for the main rotor of a helicopter, is in fact a leaf, and the outer ends, with respect to the axis of rotation of the rotor, of the leaf and of the sleeve which surrounds it are joined to the end of the current part or to the foot of the corresponding blade, whereas the inner end of the tubular sleeve is, as in the above mentioned patent, fixed to a rigid ring on which the lever controlling the blade pitch is externally articulated. The position of the flapping axis of each blade is defined, in the corresponding flexible leaf, by spherical ball-joint elements, made preferably from stratified elastomer, which are inserted respectively between the two large flat faces of the flexible leaf and the rigid ring, so that the center of the spherical ball-joint is situated substantially on the longitudinal axis of the flexible leaf. In addition, as in the first French patent mentioned, drag frequency adapters or dampers damping the oscillating movements of the blades under drag conditions, formed preferably by elastomer plates with great stiffness and high hystersis, are associated respectively with the spherical ball-joint elements, these plates being either inserted respectively between the two large flat faces of the flexible blade and the corresponding spherical ball-joint elements, or shaped so as to be incorporated in the stratified elastomer spherical ball-joint elements. In the first cast, the spherical ball-joint elements are disposed between two substantially concentric annular supports, one of which, in an external radial position with respect to the axis of the flexible leaf, is fixed to the rigid ring connected to the inner end of the sleeve, whereas the other, in an internal radial position, is connected by the visco-elastic plates of the frequency adapters to rigid support plates which are not fixed to the hub but to the flat faces of the internal end, with respect to the axis of rotation of the rotor, of the corresponding flexible leaf providing connection of the corresponding blade to the hub.
In order to facilitate this connection, each flexible leaf may be extended, beyond the rigid ring fixed to the corresponding tubular sleeve, in the direction of the rotor shaft, by a thicker terminal element, by means of which the leaf is connected to the rotor hub, preferably by means of two pins passing through two coinciding bores in the terminal element of the leaf and radial extensions of the hub.
In the case of a rotor having a number of pairs of blades opposite each other, it is advantageous for the same number of flexible and torsible leaves to be disposed so that their median superimposed parts are immobilized in the case carried by the upper end of the rotor shaft, so as to form the hub therefrom, the two free ends of each leaf, external to the case, forming respectively the radial leaves whose outer ends are joined respectively to the feet of the blades of the same pair.
When each of the rotor blades is formed by a longitudinal member in the form of a leaf, surrounded by a profiled shell, it is then advantageous for the outer ends, with respect to the axis of rotation of the rotor, respectively of each leaf and of the tubular sleeve which surrounds it to be joined respectively to the inner ends of the longitudinal member and of the profiled shell of the corresponding blade.
Each tubular sleeve and the profiled shell of the corresponding blade may be made as a single piece and each leaf and the longitudinal member of the corresponding blade may also be made as a single piece. But it is also possible for the outer end of each leaf to be joined to the foot of the corresponding blade through at least one pivot, with its axis substantially perpendicular to the plane of the rotor.
Furthermore, from the French patent application No. 85 19418 of the applicant, a rotation damper is known having a stator and a rotor which are coaxial, two end faces transversal to the common axis of the stator and the rotor are disposed respectively on each side thereof, a man chamber filled with a hydraulic fluid and defined between the end faces and the stator and the rotor, at least one vane, but preferably several vanes arranged on the stator and rotor dividing the main chamber into several volumes, means for laminating the hydraulic fluid through the different volumes of the main chamber, and at least one auxiliary chamber in communication with the main chamber, each auxiliary chamber being at least partially defined by a resiliently deformable membrane for counterbalancing the variations of volume of the fluid as a function of its temperature.
In the case where the rotation damper has only a single auxiliary chamber, this latter is advantageously defined between one of the end damper faces and a resiliently deformable membrane and is associated with this end face, which is formed with at least one passage for commnication between the auxiliary and main chambers.
However, a resiliently deformable membrane may be associated with each end face of the damper, so as to define two auxiliary chambers disposed respectively on each side of the stator and rotor.
In both cases, an end face associated with the membrane is formed of a first crown associated with the stator and a second crown associated with the rotor, the communication passage between the first main chamber and the auxiliary chamber being formed by a clearance provided between these crowns. The membrane has advantageously an annular shape and its internal and external annular peripheries are surrounded respectively by an inner ring and an outer ring, to which the membrane is fixed by vulcanization. Such a membrane surrounded by its inner and outer rings, may be advantageously housed in a corresponding annular groove defined by the two crowns of the corresponding end face. In this latter case, the inner and outer rings of a membrane are preferably adhered to the facing walls of a groove.
The resiliently deformable membrane may be an elastomer membrane or may be formed by alternately disposed layers of elastomer and metal layers. In this latter case, the layers are advantageously disposed parallel to the axis of rotation of the rotor and of the stator.
In such a rotation damper, the damping is provided by laminating the hydraulic fluid, which is preferably a high viscosity silicon oil, between the ends of the vanes and the facing cylindrical wall, when the rotor is rotated with respect to the stator about their common axis, in a mode of operation which is well known and described not only in the above mentioned French patent but also in French Pat. No. 2 356 347.
Finally, it should be noted that the French patent application No. 85 19418 proposes the application of the rotation damper which forms the subject matter thereof to a vehicle suspension as well as to a manual control comprising a control handle fixed to a shaft and whose angular movement is representative of an output signal driving a member, for example an aircraft flight control, for orientating by means of an electric jack the front bogie of the tricycle landing gear of a helicopter.
The present invention provides then an improvement to variable pitch rotors and rotor hubs, particularly for helicopters, of the type described in the French Pat. Nos. 2 397 325 and 2 397 327 of the applicant, by improving the structure of the combined bearing devices with which they are equipped so as to reduce the power required for controlling the angle of attack of the blades.
Another object of the invention is to improve the combined bearing devices of rotors and rotor hubs of the above type, so that they provide damping of the angular oscillations of the blades under drag conditions which is sufficient, in all the different cases of application envisaged for low or medium tonnage helicopters.
Yet another object of the invention is to provide an improved combined bearing device, having a new type damper for damping the angular oscillations of the blades under drag conditions and which, with respect to a conventional visco-elastic damper such as known from French Pats. Nos. 2 397 325 and 2 397 327 has the main advantage of dissociating, in the same member, the stiffening function (and so the resilient return which is all the higher the greater the stiffness) and damping function, which facilitates the optimization of each of these functions.